Programmed cell death, or apoptosis, is important in homeostasis of the immune system:for example, non-functional or autoreactive lymphocytes are eliminated through apoptosis. One member of the tumour necrosis factor receptor (TNFR) family, Fas (also known as CD95 or Apo-1), can trigger cell death and is essential for lymphocyte homeostasis [1,2]. FADD/Mort1 [3-6] is a Fas-associated protein that is thought to mediate apoptosis by recruiting the protease caspase-8 [7,8]. A dominant-negative mutant of FADD inhibits apoptosis initiated by Fas and other TNFR family members [6,9-14]. Other proteins, notably Daxx, also bind Fas and presumably mediate a FADD-independent apoptotic pathway . Here we investigate the role of FADD in vivo by generating FADD-deficient mice. As homozygous mice die in utero, we generated FADD sup -/- embryonic stem cells and FADD sup -/- chimaeras in a background devoid of the recombination activating gene RAG-1, which activates rearrangement of the immunoglobulin and T-cell receptor genes. We found that thymocyte subpopulations were apparently normal in newborn chimaeras. Fas-induced apoptosis was completely blocked, indicating that there are no redundant Fas apoptotic pathways. As these mice age, their thymocytes decrease to an undetectable level, although peripheral T cells are present in all older FADD sup -/- chimaeras. Unexpectedly, activation-induced proliferation is impaired in these FADD sup -/- T cells, despite production of the cytokine interleukin (IL)-2. These results and the similarities between FADD sup -/- mice and mice lacking the beta-subunit of the IL-2 receptor suggest that there is an unexpected connection between cell proliferation and apoptosis.